The present invention relates broadly to dual purpose steam turbine power plants which cogenerate energy for electrical consumption and industrial process heating needs, wherein an industrial process may extract steam from the turbine for use in one or more heat exchanger sections employed thereby, and more particularly, to a heat exchanger section operative to control the pressure of the extracted turbine steam substantially at a predetermined pressure set point value.
In most steam turbine power plants proposed to have a dual purpose, after the steam has been conducted through the steam turbine wherein energy is extracted to mechanically power the steam turbine to drive a generator coupled thereto to produce electrical energy at a desired power, the exhausted steam or portion thereof may be provided to one or more heat exchangers of an industrial process, like desalinization, for example, for the purposes of extracting heat energy therefrom. As a result of this dual purpose configuration, the safety requirements and operational needs of both the electrical power plant and industrial process must be considered. One parameter of prime importance is that of the steam turbine exhaust pressure or more commonly termed the turbine back pressure. It is well known that in a conventional single purpose steam turbine plant, if this back pressure is allowed to exceed some predetermined limiting value, it may result in mechanical overloading of the last stages of the low pressure turbine element especially those stages in close proximity to the turbine steam exhaust. As a safety requirement in most of these single purpose plants, either the turbine is tripped as a result of a back pressure excursion beyond the preset limit or the back pressure is limited to remain below the preset limit by some control method exemplified by that described in the U.S. Pat. No. 4,004,424 issued to Maddagiri on Jan. 25, 1977. It is also well known that in order for an industrial process, which may be interfaced to a steam turbine power plant, to efficiently utilize the extracted low pressure steam provided thereto from the steam turbine exhaust, for example, the pressure of this extracted steam, or in effect, temperature thereof, should be at a value to insure the economic commercial feasibility of the industrial process; otherwise, the dual purpose of the power plant would not be warranted.
Therefore, it is evident that an optimum process heat exchanger pressure should be derived to satisfy both the safety requirements of the steam turbine and the efficiency requirements for commercially feasible operation of the industrial process interfaced therewith. And accordingly, in order to preserve the availability of the electrical power generation of the steam turbine-generator and ensure commercially efficient operation of the industrial process as related to utilization of the extracted steam from the steam turbine, it is of paramount importance to continuously maintain the steam pressure of the heat exchanger at the aforementioned derived optimum value during the concurrent dual operation of the electrical generation and the industrial process. It is a primary object of the present invention as described hereinbelow to accomplish this pressure control pertaining to the dual purpose operability of the steam turbine power plant.